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Does the thermostat have batterys in it if not that is your questions those 4 wire digitals are crap they do not have a common on them so they "leak" a little voltage through the control circuits and can make a furnace do some wierd stuff seen it several times with the Honeywell T8400 series and Lennox equipment.

first thought to come to mind is , check if the pressure proving switch is opening up. this could explain if your not getting any error code on the LEDs. if so check the flue for blockage or maybe time for a high wind kit.

Originally posted by jstarzy Can anyone give me an idea of what is causing the furnace to cycle the gas feed to the burners on for 10 seconds, then off for 10 seconds for the duration of time the thermostat calls for heat?

It always helps to have the model and serial number when discussing this stuff. This is not the same furnace that everyone was talking about on the first page.

I have seen inducer assembly and pressure switch problems in some of the G16s. Chances are it was not properly vented. They should of used class B flue pipe and only vented it into the chimney under certain circumstances. To be fair, this was not known in the late 80's (so I'm told. I've been doing this 4+ years). I wish your tech well in ordering those parts. I have had trouble getting a G16 pressure switch.

The gas co tech should have verified that there wasn't a chimney related problem before swapping parts...It is not too difficult to verify that the chimney is ok. sounds to me like he's hoping and not really testing things like he should be...maybe you should request another service tech to diagnose.. let us know how it works out

All...thanks for help so far....here are some more observations while I wait for the parts...

Chimney had a healthy amount of vapor comoing out of it this morning that I could see from driveway

Chimney is brick with two flues....2/3 for fireplace, 1/3 for furnace and water heater. I do not know how it is lined. House was built in 1969. Both flues are capped at top of chimney to keep out the wildlife. Chimney is approx 22-26 feet high from basement to top of brick (2 story center hall.

There is a 6-8 ft horizontal (slight pitch up) pipe from furnace to the penetration in the block in the wall that leads to the base of flue in basement. It seems to be about as old as the furnace or newer (metal still shiney).

How water heater has slightly melted blue/red washers at in and out lines at copper connecting point on top. This always puzzled me. There is some residual heat at exhaust port of HWH just where the "cap" sits on the HWH exhaust. This seems to have melted the washers. However...there is no problem with any gasses escaping around this flue...just the residual heat that leads to the piping to the flue. I have a CO detector in basement and all is fine. Could this be an indication of poor drafting? I do not recall that much residual heat around the same type unit in my last house.

So...if the chimney is NG...why..an obstruction? Or...dirty..or...is it a bad match between equipment and flue (OUCH!). What to do? How do you measure the draft? Should gas co. have this equipment? Can I jury-rig some metal flex and use the fireplace flu to trouble shoot? (Access door in basement, this flue has excellent draft)

I am debating replacing this 15+ year old furnace with 91% unit and hi-efficiency HWH...I'll get some plastic out to the garage roofline and forget about the chimney all together...thoughts?

How long are these G16s good for? The firebox and exchanger are in excellent shape.

Thanks for any follow up. Sorry for the saga..but a good learning experience, I guess.

Jim S.
Central New Jersey (about 11f last night with snow on the roof!)